Observe the graph :

The slope at the left of the graph is positive.This indicates that the sales initially increase.

Next, the slope is negative, indicated that the sales are the decreasing.

Then, the slope increases again at the same rate, indicating the sales are increasing at a steady rate.

Then the slope is zero for a short period of time. This indicates that there are no sales during this time.

Then, the slope increases but a a slower rate than before. This indicates that the sales are increasing but at a slower rate.